Portal

HI all,A couple of weeks ago all of the sudden Portal says it has to be reinstalled, yet I see the files are still there. I don't want to download the whole thing again. I clicked install but sure enough it is asking to download 2+ gigs all over again. :/

There's an ACF file that tells Steam if a game is installed or not, located here: Steam\SteamApps. If this file is corrupted or deleted, Steam will try to redownload the game completly even if the files are still there.